Scientists spot sleeping jellyfish

The suite of genes that induce and regulate sleep has not yet been fully uncovered, but researchers at the California Institute of Technology in the U.S. have demonstrated that they are remarkably ancient, and strongly conserved across the animal kingdom.

To make this finding, Sternberg and his colleagues had to first come up with a workable definition of what sleep might look like in a jellyfish.

"It's the first example of sleep in animals without a brain", says study coauthor Paul Sternberg, PhD, a Howard Hughes Medical Institute (HHMI) Investigator at the California Institute of Technology, in a release. The findings push the origin of sleep further down the evolutionary tree of life - to before the emergence of a centralized nervous system. Every complex animal, from the humblest fruit fly to the largest blue whale, sleeps - yet scientists can't explain why any organism would leave itself vulnerable to predators, and unable to eat or mate, for a large portion of the day. Theories involve everything from memory, to learning, to cellular recovery.

The goal and evolutionary origins of sleep are among the biggest mysteries in neuroscience.

It's a controversial subject. He'd like to study sponges, or even single cell protozoa, next and perhaps push the origin of sleep back even further. Researchers do know that the same genes and molecules that control sleep in worms and flies also regulate sleep in zebrafish and humans, Nath said. But not everyone's convinced.

"But jellyfish are the most evolutionarily ancient animals known to sleep".

Cassiopea jellyfish live in clear, shallow, tropical waters of the Pacific and western Atlantic oceans, eating plankton. They raised the jellies gently up from the bottom of the tank, then rapidly yanked the container downward, leaving the jellyfish suspended in the water.

The unusual jellyfish resemble cauliflower or an upside-down mushroom, with their round, sand-dollar-sized bodies resting on the ocean floor and their tentacles curled upwards. The researchers noted that for periods during the night, the pulse rate dropped to 39.

Ravi Nath, a Caltech graduate student and a co-author of the new study, typically studies this sleep-like state in C. elegans. They found that when they disturbed the jellyfish this way during the last 6 hours of the night, the jellyfish showed a 12 percent decline in pulsing in the first 4 hours of the next day, as if they were having trouble waking up. The researchers then created an image processing program to count the pulses of 23 jellies over six consecutive days and nights. When the researchers put a little midnight snack in the water column, the jellies perked up and started pulsing at daytime rates, indicating that this quiescent period was easily reversible.

"It's like the odor of coffee permeating your consciousness in the morning", Sternberg says.

"This work provides compelling evidence for how early in evolution a sleep-like state evolved", says Dion Dickman, a neuroscientist at the University of Southern California in Los Angeles.

Researchers spotted a second sign of sleep after dropping the floor out from dozing jellies. During the day, Cassiopea will quickly pulse their bells and swim down to rest on the mesh again. Ms. Bedbrook started to believe they were onto something. This delayed response to stimulation is typical for sleeping animals.

These criteria were devised by examining sleep in other animals, including humans. Bedbrook said. "What do they all have in common that could be the reason these animals go through this sleep state?" The findings involving such a primordial creature raise fresh questions about sleep's origin and objective. Animals as simple as the worm Caenorhabditis elegans, which has just 302 neurons and an extremely simple central nervous system, have been found to exhibit patterns of activity and rest that look an bad lot like sleep.

"This work shows that sleep is much older than we thought". He hopes his future studies can answer more complicated questions.